Recently, communication systems using the frequency band ranging from several hundreds MHz to several GHz or an extra higher and wider frequency band, i.e. ultra wide band (UWB), has been gaining in popularity, and distance measuring systems using the signals of UWB have been studied progressively.
The ultra wide band communication differs from conventional wireless communications in using narrow pulses (sometimes referred to as short pulses) and spreading frequency components in the frequency bands ranging from several hundreds MHz to several GHz or an extra higher and wider frequency band for communication. The distance measuring system using the UWB communication calculates a distance by measuring a time difference between a short pulse signal transmitted and the short pulse signal received. It is necessary to control pulses having extremely narrow width, such as not greater than 1 nanosecond, for a high-speed communication using UWB or measuring a distance accurately, yet it has been difficult to carry out this control. The recent progress in semiconductor technology, however, allows carrying out this control. Advantages of the UWB communication are these: Use of short pulses allows many users to use signals of frequencies common to the users because the signals scarcely overlap each other per unit time, so that respective communications can be separated with ease and the communications can be done in a concurrent manner. The UWB communication also spreads frequency components over an extremely wide frequency band, so that the communication hardly invites noises or radio-wave interference caused by a specific frequency.
One of pulse generating circuits of conventional transmitters is disclosed in Japanese Translation of PCT Publication No. 2003-513501, and one of conventional receivers can employ the demodulating circuit disclosed in U.S. Pat. No. 6,452,530 or a signal modulation circuit, which improves an S/N ratio, disclosed in Unexamined Japanese Patent Publication No. H10-190356.
FIG. 14 shows the pulse generating circuit of the conventional transmitter disclosed in Japanese Translation of PCT Publication No. 2003-513501. In FIG. 14, the pulse generating circuit of the conventional transmitter comprises the following elements:
analog waveform generating circuit 801 which generates any signal of analog waveform;
inductor 802; and
circuit 803 formed of negative resistance elements having a stable region and an unstable region.
The analog waveform signal including transmission data is generated by analog waveform generating circuit 801, and then fed into inductor 802, which converts the waveform of the signal. The signal having undergone the waveform conversion is then fed into circuit 803 formed of the negative resistance elements. Circuit 803 changes its operating status in the stable region and the unstable region in response to the analog waveform signal having undergone the waveform conversion, so that circuit 803 oscillates in the unstable region. Inductor 802 converts the waveform of the signal so that circuit 803 can oscillate in the unstable region. The oscillation in the unstable region allows dividing one pulse of the signal into plural short pulses, so that a transmission output signal is obtainable.
FIG. 15 shows the pulse signal demodulating circuit of the conventional receiver disclosed in U.S. Pat. No. 6,452,530. In FIG. 15, the pulse demodulating circuit comprises the following elements:
receiving unit 901 for converting pulse signals received from antenna 904 into analog signals;
reception pulse generating circuits 902 having different pulse generating standards from each other; and
composition determining circuit 903 for generating a reception data signals by arranging the pulse signals generated by respective pulse generating circuits 902.
The pulse signal demodulating circuit converts the received pulse signals into analog signals at receiving unit 901, and the converted analog signals are fed into reception pulse generating circuits 902 having different pulse generating standards from each other. Composition determining circuit 903 arranges the pulse signals generated by the respective pulse generating circuits 902 for generating a reception data signals.
FIG. 16 shows the signal demodulating circuit which can improve the S/N ratio of the conventional receiver disclosed in Unexamined Japanese Patent Publication No. H10-190356. In FIG. 16, delay circuit 1001 produces a delay time of τf=nτc, where τf indicates delay time “τ” (second) and τc indicates iterative cycle (n=0, 1, 2, . . . ) of the modulated frequency when the signal is FM modulated. Adaptation of the delay detection to the demodulation of FM modulated wave allows composite circuit 1002 to add up modulated signals, i.e. repetitive waves, and not to add up noise components, i.e. non-correlative waves. This mechanism increases the ratio of desirable wave vs. noise components, so that the S/N ratio can be improved.
Although this is not described with an example, use of a different spread-code for respective users in the spread spectrum communication allows overlapping plural users' signals in one frequency band (CDMA: code division multiple access). This CDMA is employed in the mobile communication.
When both of a mobile station near a base station and another mobile station distant from the base station communicate with the base station simultaneously, the radio wave from the nearby mobile station attenuates only a little because of a short distance from the base station, so that the radio wave arrives at the base station as a large signal. On the other hand, the radio wave from the distant mobile station attenuates by a greater amount because of a long distance from the base station, so that the radio wave arrives at the base station as a small signal. In this case, the base station adjusts the power level of the receiving system in accordance with the large signal transmitted from the nearby mobile station, so that the small signal transmitted from the distant mobile station cannot be demodulated, i.e. a distance problem occurs. To overcome this distance problem, the transmission power of the nearby mobile station is lowered, which is known as a method of transmission power control. This method allows the mobile station to control the transmission power based on the control information supplied from the base station, so that the difference in signal powers of the radio waves transmitted from the nearby mobile station and the distant mobile station can be reduced. As a result, both of the signals can be adjusted to be received and demodulated in terms of the power level of the receiving system.
When the conventional transmitter and receiver disclosed respectively in Japanese Translation of PCT Publication No. 2003-513501 and U.S. Pat. No. 6,452,530 receive radio-wave from a distant wireless device and another radio-wave from a nearby wireless device (a wireless device is used as a generic term of a transmitter and a receiver) simultaneously, the reception power level is controlled in response to the signal of greater power due to the distance problem discussed above, so that the signal of smaller power sometimes cannot be received or demodulated. On top of that, those signals sometimes interfere with each other, so that the signals can be processed erroneously.
The technique of improving the S/N ratio of the conventional receiver disclosed in Unexamined Japanese Patent Publication No. H10-190356 utilizes the phenomenon that signals of regularity can be superimposed, and signals of non-regularity cannot be superimposed, so that signals of regularity supplied from plural wireless devices can be hard to be separated. The technique of transmission power control is effective in the communication between one station and multi-stations, so that when plural wireless links are available, a complicated wireless control across the links is needed. For instance, assume that wireless devices communicate with each other at long range, and other wireless devices communicate with each other at short range, then it is difficult to lower the transmission power of the wireless devices involved in the long range communication because the power level is adjusted in accordance with the receiving level of the wireless devices involved in the short range communication.
On top of that, use of the time-division multiplexing, frequency-division multiplexing or code-division multiplexing as a communication method can make the wireless device complicated, upsized and expensive.